(1) Field of the Invention
The present invention relates to a method and device for detecting the loss-of-signal condition at the input of a transmission line interface, said input signal being a coded one and comprising pseudo-random sequences of signal transitions.
(2) Description of the Prior Art
In on-line transmission systems, the hardware line interface unit (LIU) receiving signals from the lines (cable-type) can be too sensitive and can locally regenerate a signal even in the absence of useful signal transitions on the line, especially when the signal on the line becomes too low, under a given threshold, or in the absence of signal because of, e.g., a break of the cable, since the high input sensitivity leads to an erroneous interpretation of the line and apparatus noise levels as useful signal, thus regenerating the input signal as a sequence of pseudo-random transitions.
Under these conditions, the detection of loss of input digital signal (LOS), which is normally present, becomes unreliable and LIU regenerates an erroneous signal mixed with noise.
In order to overcome this problem, various hardware arrangements, based on an input circuit which detects the signal level followed by a signal muting stage, are already known.
Normally, said additional circuit is bulky, expensive and further requires to redesign and replace the entire input printed circuit board (PCB) containing the line interface LIU for all the apparatuses already installed, should they be not provided with said function, and it was necessary to connect it afterwards.
Therefore, it is an object of the present invention to overcome the aforesaid drawbacks and provide a method and device for detecting the loss-of-signal LOS condition at the input of a transmission line interface.
The present invention provides a method and device for detecting the loss-of-signal LOS condition at the input of a transmission line interface consisting in a very simple additional procedure to be inserted, during the decoding step, into the line interface when the input signal is coded. In fact, the decoder itself can be used as a loss-of-signal condition detector, since the pseudo random sequence of input signal transitions includes code violation sequences. The additional procedure over a certain threshold error rate, corresponding to a number of code violations per unit of time, detects the loss-of-signal, LOS, condition.
The present invention provides said method and device for detecting the loss-of-signal (LOS) condition at the input of a transmission line interface as best described below and claimed in the following claims, which are an integral part of the present specification.
The method according to the invention has the basic advantage that it can be carried out by means of a software routine which can be easily inserted in all the already installed apparatuses having no hardware arrangements installed yet, instead of replacing the entire card. In fact it suffices to reprogram the program storage in any manner known per se.
Further objects and advantages of the present invention shall become clear from the following detailed description of an embodiment thereof and from the accompanying drawings which are attached merely by way of a non limiting example.